This invention relates to a novel cathode-ray tube and to a novel method for producing a getter metal film in a cathode-ray tube.
In order to maintain an adequate vacuum in an apertured-mask cathode-ray tube, it is the practice to produce a thin film of barium metal on inner surfaces of the tube envelope and on the electron-beam-receiving surfaces of the mask. Such a film may be produced by exothermic or endothermic reactions or both as described for example in U.S. Pat. Nos. 3,381,805, 3,385,420 and 3,389,288. The barium metal films getter, or capture, residual gases in the tube at the time of film deposition and subsequently during the operating life of the tube. A barium-metal-coated mask emits a continuous spectrum of X-rays when bombarded by electron beams, which spectrum is characteristic of a barium target. As the apertured-mask cathode-ray tube art has developed, higher acceleration voltages (above 20,000 volts) and greater electron-beam currents are being used. As a result, more X-rays are being generated with consequent hazard to the viewer.
The present invention is based on the realization that, while strontium metal films can getter residual gases as predicted, they generate fewer X-rays than barium metal films when bombarded by electron beams. Strontium metal also possesses properties which provide additional valuable advantages both to the method of producing the getter film and to the tube employing the getter metal film. Thus, the novel tube comprises a strontium metal film on at least a portion of the electron-beam-receiving surface of the apertured mask of the tube.
Strontium metal has a vapor pressure of about 100 torr at about 1107.degree.C, whereas barium requires a temperature of 1430.degree.C (323.degree.C higher) to achieve the same vapor pressure. A vapor pressure of at least 100 torr is what is required for producing a getter metal film of necessary thickness in a vacuum. Thus, in the novel method a mass comprising a strontium-aluminum alloy containing about 40 weight parts strontium metal and about 57 to 63 weight parts aluminum metal is sealed into a cathode-ray tube, and then heated above 1100.degree.C until a substantial portion of the strontium is liberated and vaporized from the mass. The vaporized strontium condenses as a metal film on internal surfaces of the tube. As compared to producing a barium film, lower temperatures may be used for producing the strontium metal film, resulting in fewer reject tubes. A gas-releasing material may be included in the mass for providing a predetermined gas pressure in the tube during the liberation of strontium from the mass. The gas pressure may be used to modify the distribution of the strontium on the internal surfaces of the tube.
Where the strontium metal film resides on surfaces that are struck by primary electron beams, a lesser amount of X-radiation is generated than with barium metal surfaces. Also, since strontium metal films absorb less energy from impinging electron beams, a luminescent target coated with a getter metal film is excited to a greater brightness for the same power input than with a barium metal film of the same thickness.